1. Field of the Invention
The present invention relates to print styli for an electrosensitive printer, and more particularly to print styli which are not subject to fouling from print residue.
2. Description of Prior Art
In an electrosensitive printer which prints on electrosensitive paper, a mark is produced on the paper by passing an electric current from print styli through the paper. A plurality of the print styli are mounted on a nonconductive bar and separated from each other by nonconductive spacers. The spacers perform the functions of electrically isolating the print styli from each other and of providing a clearance between adjacent print styli. Thus, each print stylus can be fired individually to command, producing an individual dot on the paper. Characters are formed by firing the appropriate print styli in a dot row, advancing the paper, and printing the next dot row. This process is repeated until the characters are completely formed.
The print styli are spring loaded against the electrosensitive paper so that adequate contact is maintained between the print styli and the paper even over paper discontinuities such as perforations. A portion of the circumference of a print finger is used as the print stylus. During the print process a residue is formed both above and below the print stylus. The residue is forced between the print stylus and the print finger by the motion of paper and by gravity. This leaves the print stylus open to contamination from the print residue which eventually surrounds the print stylus to immobilize it.
FIG. 1 shows a print finger similar to one used by Baker, U.S. Pat. No. 3,713,168, and Walker et al, U.S. Pat. No. 3,961,336. The print stylus is retained by a mechanism which allows print residue to be forced down into the print finger by the motion of the paper and gravity. Print residue starting from position A would be forced passed the print finger and would fill the void between the movable arm and the body of the print finger (positions B and C). Thr residue would accumulate in the area of the print stylus retainer (positions C and D), fouling the print stylus, i.e., preventing the print stylus from returning to its static position after being depressed. The retainer is necessary to prevent the movable arm from being bent past its yield point when brushed in the direction opposite the paper motion during cleaning.